1. Field of the Invention
The present invention relates to a semiconductor device.
2. Description of Related Art
In recent years, PRAM (Phase change Random Access Memory) has increasingly gained popularity as one of non-volatile memories which hold stored data even if they are not supplied with power. The PRAM stores data by using resistance of a phase change material that changes corresponding to a phase change of the phase change material due to the temperature (whether the material is in a crystallized state or an amorphized state).
A phase change film, which is formed of a phase change material such as a compound and that is called a “GST film” which contains germanium (Ge), antimony (Sb), and tellurium (Te), is employed in the PRAM. The phase of the GST film alternates between a non-crystal state and a crystal state, corresponding to Joule heat generated by a current supplied thereto. This phase change causes a change in resistance RGST of the GST film.
In general PRAMs, for changing resistance RGST of a GST film in a memory cell, write circuit YYj is used by being connected to the other circuit through transistors Q10 and Q20 as illustrated in FIG. 1. For reference, in FIG. 1, transistors Q10 and Q20 are P-ch (P channel) transistors, whose carriers are holes.
Controlling write current IWR is performed by changing write current IWR corresponding to gate voltage VREF generated through resistor division of drive voltage VDD, which is applied to a gate electrode of transistor Q10 connected to write circuit YYj. In this example, one value from among divided resistance values is selected by inputting a resistance value selection signal (for example, SEL0 is the resistance value selection signal for selecting a predetermined resistance value) to select respective divided resistance values.
In this regard, an internal voltage generator circuit is employed for generating and outputting gate voltage VREF, in order to apply gate voltage VREF to the gate electrode of transistor Q10 connected to write circuit YYj. An internal voltage generator circuit utilizing a comparator is also contemplated as such a kind of general internal voltage generator circuit. One example of such internal voltage generator circuits is disclosed in JP-2005-174351A.
In a general technique for controlling the write current, as shown in FIG. 1, a constant voltage generated by the internal voltage generator circuit is used as gate voltage VREF which is applied to the gate electrode of transistor Q10.
When gate voltage VREF applied to the gate electrode is a constant voltage, a problem arises in that the write current flowing from a source electrode to a drain electrode of transistor Q10 is affected by the process employed when transistor Q10 is manufactured, fluctuations in drive voltage VDD for use in the generation of gate voltage VREF, and the temperature of transistor Q10.